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The present invention relates to coatings that are able to evolve heat when connected to a source of electricity (electrothermic coatings) and more particularly to electrothermic coatings that utilize non-metallic particles for achieving remarkable heating characteristics.
The art has proposed xe2x80x9celectrically conductivexe2x80x9d coatings utilizing metallic particles for anti-static applications. Coatings based on non-metallic particles even appear in the literature. These coatings, however, typically only generate low amounts of heat and often break down the binder when asked to achieve moderate to high temperatures (say, in excess of around 100xc2x0 C.). Nevertheless, the art is replete in such exothermic coatings teachings.
Namura (U.S. Pat. No. 5,549,849) proposes a combination of graphite particles, metal particles, and carbon black to prepare conductive coatings. Miller (U.S. Pat. No. 6,086,791) proposes a non-metallic electrically conductive coating composition effective in emitting heat without break-down when connected to a source of electricity, which coating composition is made from a binder; electrically conductive flake-like carbon black particles ranging in size from between about 5 and 500xcexc; electrically conductive flake-like graphite particles ranging in size from between about 5 and 500xcexc; and a volatile solvent. The weight amount of flake-like carbon black particles and flake-like graphite particles together ranges from between about 10 and 75 weight-% based on the non-volatile solids content of the coating composition.
The present invention solves many of the problems encountered in the art in formulating non-metallic electrothermic coatings.
A coating composition, effective in emitting heat without breaking down when connected to a source of electricity, is formulated from a binder; an electrically conductive carbon black particle generated by high temperature pyrolysis of acetylene and having a particle size between of between about 5 and 500xcexc; an electrically conductive graphite particle having a degree of crystallinity of at least about 67% and having a particle size between about 5 and 500xcexc; and a volatile solvent. The weight amount of (b) and (c) together ranges from between about 10 and 75 weight-% based on the non-volatile solids content of the coating composition.
The novel coatings are made conventionally by initially forming a pigment grind and then letting down the grind in additional solvent with the incorporation of additives as is necessary, desirable, or convenient. The binder should be able to withstand the expected temperatures of the coating and, thus, should be temperature resistant silicone resins, polyamide resins, bis-maleimide resins, and the like.
In another aspect of the present invention a metal oxide particle is added to the coating composition. When such a coating is drawn down as a film, cured, and electrically energized, the film glows, i.e., emits IR energy.
Advantages of the present invention include the ability to generate temperatures ranging up to around 600xc2x0 F. Another advantage is the ability to produce a self-regulating temperature coating. A further advantage is that the inventive coating maintains its coating properties and can be applied, inter alia, by brush, roller coat, reverse roller coat, spray, and the like. These and other advantages will be readily apparent to those skilled in this art.